Moles, mass and molar relationships
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Moles, mass and molar relationships — Free MYP4 Chemistry Practice Questions

1QuestionConverting between mass and molesConcept Practice
3 marks~5 minCriterion A
The graph below shows the mass of magnesium ribbon (g) against time (s) as it reacts with excess hydrochloric acid. The magnesium starts at 1.00 g1.00\ \text{g} and its mass approaches 0.00 g0.00\ \text{g} after approximately 120 s120\ \text{s}.
a
Describe the trend shown in the graph. [1]
b
Explain why the rate of reaction decreases over time. [1]
c
Construct the balanced chemical equation for this reaction, including state symbols. [1]

Solutions

2QuestionConverting between mass and molesAssessment Practice
4 marks~6 minCriterion C
When magnesium burns in oxygen, the reaction follows the equation

2Mg+O2→2MgO.2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}.

Molar masses: Mg=24.3 g mol−1\text{Mg} = 24.3 \ \text{g mol}^{-1}; O=16.0 g mol−1\text{O} = 16.0 \ \text{g mol}^{-1}; MgO=40.3 g mol−1\text{MgO} = 40.3 \ \text{g mol}^{-1}.
a
Calculate the mass of magnesium required to produce 60.0 g60.0 \ \text{g} of MgO. [2]
b
A student claims: "Because Mg and MgO are in a 1 : 1 molar ratio, the mass of Mg needed always equals the mass of MgO produced." Using your calculated values, explain why this claim is incorrect and identify the chemical reason for the mass difference. [2]
Question diagram

Solutions

3QuestionUsing formula: moles = mass ÷ molar massConcept Practice
2 marks~3 minCriterion B
The table below shows the first five straight-chain alkanes and their molar masses.

AlkaneMethaneEthanePropaneButanePentane
Molecular formulaCH4CH_4C2H6C_2H_6C3H8C_3H_8C4H10C_4H_{10}C5H12C_5H_{12}
Molar mass (g/mol)1630445872
a
Deduce the molar mass of hexane, C6H14C_6H_{14}, without a periodic table, showing your reasoning. [1]
b
Explain why each successive alkane in this series has a molar mass exactly 14 g/mol greater than the previous one. [1]

Solutions

4QuestionInterpreting coefficients in chemical equationsAssessment Practice
6 marks~9 minCriterion D
A factory burns methane (CH4_4) in excess oxygen each day:

CH4+2O2→CO2+2H2O\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}

The factory consumes 160 kg of CH4_4 daily. The local authority requires the factory to report its daily CO2_2 output and assess its environmental significance.
a
Show that the mass of CO2_2 produced daily is 440 kg. [2]
b
Explain how the daily CO2_2 emissions from this factory contribute to the greenhouse effect. [2]
c
Evaluate the reliability of using this balanced equation alone to predict the factory's actual daily CO2_2 emissions. [2]
Question diagram

Solutions

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